The interaction is very significant and has been known to occur with even very small amounts of alcohol. It's not clear why metronidazole and alcohol affect each other, and it's impossible to be sure exactly how long the problem might last after stopping metronidazole. This is because there has been very little research into this.

However, the advice given by one of the manufacturers of metronidazole is that you should avoid drinking alcohol while taking it and for 48 hours after finishing the course. This makes sense because 48 hours is roughly the time it takes for metronidazole to be cleared from the body of an average adult.

During this time, it may be sensible to also avoid cough and cold remedies and mouthwashes that contain alcohol.